Continuous synthesis of high-entropy alloy nanoparticles by in-flight alloying of elemental metals

Abstract

High-entropy alloy (HEA) nanoparticles (NPs) exhibit unusual combinations of functional properties. However, their scalable synthesis remains a significant challenge requiring extreme fabrication conditions. Metal salts are often employed as precursors because of their low decomposition temperatures, yet contain potential impurities. Here, we propose an ultrafast (< 100 ms), one-step method that enables the continuous synthesis of HEA NPs directly from elemental metal powders via in-flight alloying. A high-temperature plasma jet ( > 5000 K) is employed for rapid heating/cooling (10³ - 10⁵ K s^-1), and demonstrates the synthesis of CrFeCoNiMo HEA NPs ( ~ 50 nm) at a high rate approaching 35 g h^-1 with a conversion efficiency of 42%. Our thermofluid simulation reveals that the properties of HEA NPs can be tailored by the plasma gas which affects the thermal history of NPs. The HEA NPs demonstrate an excellent light absorption of > 96% over a wide spectrum, representing great potential for photothermal conversion of solar energy at large scales. Our work shows that the thermal plasma process developed could provide a promising route towards industrial scale production of HEA NPs.

Fig. 1. The ICPJ strategy for continuous synthesis of HEA NPs.

a Schematic of an inductively coupled plasma jet (ICPJ) process developed for the continuous synthesis of HEA NPs directly from a mixture of pure elemental metal powders via in-flight alloying. b, c Schematic diagrams illustrating the formation mechanism of (b) conventional alloy NPs and (c) HEA NPs by a thermal plasma jet.

Fig. 2. Morphological and structural characterization of HEA NPs.

a Photo of the HEA NP samples produced after a 150-min synthesis experiment. b SEM images of the HEA NPs produced with different plasma gases of hydrogen and helium, showing morphology change. Scale bar, 100 nm. c Size distribution of the HEA NPs produced with different plasma gases. d XRD pattern of the feedstock mixture of Cr, Fe, Co, Ni, and Mo. e XRD patterns of the HEA NPs (single FCC) produced with different plasma gases which confirm the in-situ alloying of pure elemental metals by the ICPJ strategy.

Fig. 3. Composition and phase stability analysis of HEA NPs.

a, b TEM, HR-TEM and HAADF-STEM images of the HEA NPs produced with different plasma gases of hydrogen and helium by the ICJP. Scale bar, 10 nm (HR-TEM) and 100 nm (HAADF). c–f EDX elemental maps of single and multiple HEA NPs, showing homogenous distribution of the five metals in particles. Scale bar, 25 nm (single NP) and 100 nm (multiple NPs). g, h EDX line scans of individual NPs showing the spatial uniformity in their compositions; (g) HEA-H₂ case and (h) HEA-He case. i, j Phase stability calculations by DFT simulation for the HEA NPs; (i) Cr_0.19Fe_0.22Co_0.22Ni_0.22Mo_0.15 (HEA-H₂ case) and (j) Cr_0.24Fe_0.23Co_0.26Ni_0.22Mo_0.05 (HEA-He case), demonstrating a higher stability of a FCC structure over a BCC.

Fig. 4. Optical emission measurements, thermofluid simulations, and homogenous nucleation temperature calculations for the ICPJ process.

a, b Optical emission spectra measured at (a) Z = 0.23 m and (b) Z = 0.49 from the top of the plasma torch for the HEA-H₂ case. c–g Thermofluid simulation showing the effect of the reactor geometry (D_r = D_t v.s. D_r = 3D_t where D_r is the reactor diameter and D_t is the torch diameter) on the turbulence intensity. h, i Calculated (h) saturation vapor pressures and (i) nucleation temperatures of each element in a vapor mix of Cr:Fe:Co:Ni:Mo = 1:1:1:1:1 produced at a feed rate of 1.5 g min⁻¹, showing the existence of a large nucleation temperature gap.

Fig. 5. Effects of the plasma gas on the HEA NP growth by the ICPJ strategy.

a Temperature fields calculated for different plasma gases of Ar (100%), Ar-H₂ (H₂: 8.3%), and Ar-He (He: 77.4%), showing different cooling rates of the plasma jet. b Thermal conductivity (κ_eff) distributions calculated which show the effect of the plasma gas on the heat transfer rate. c Temperature zones where HEA NPs nucleated are expected to be in a liquid phase (i.e., liquid zone) for Ar-H₂ and Ar-He cases. d–f Axial temperature profiles with local heating and cooling rates calculated for different plasma gases. The arrows represent the estimated liquid zones. g Residence times of HEA NPs calculated for different plasma gases. h Growth mechanisms of HEA NPs under different plasma gases of Ar-H₂ and Ar-He.

Fig. 6. HAADF-STEM images and EDX elemental maps of various HEA NPs synthesized by the ICPJ process.

a, b (a) a single and (b) multiple CrMnFeCoNi HEA NPs produced with hydrogen plasma. c, d (c) a single and (d) multiple CrMnFeCoNi HEA NPs produced with helium plasma. e, f (e) a single and (f) multiple MnFeCoNiCu HEA NPs produced with hydrogen plasma. g, h (g) a single and (h) multiple MnFeCoNiCu HEA NPs produced with helium plasma. i, j (i) a single and (j) multiple CrFeCoNiCu HEA NPs produced with hydrogen plasma. k, l (k) a single and (l) multiple CrFeCoNiCu HEA NPs produced with helium plasma. The elements in CrMnFeCoNi and MnFeCoNiCu HEA NPs have near equimolar ratios while the Cu content in CrFeCoNiCu HEA NPs (12~14%) is deviated from that of feedstock due to local segregation (Supplementary Figs. 18, 21, 24 and Supplementary Table 11). Scale bar, 50 nm (single NP) and 100 nm (multiple NPs).

Fig. 7. Optical absorption performance of the HEA NPs.

Absorptance spectra of the HEA NPs (CrFeCoNiMo) prepared by the ICPJ strategy with different plasma gases. Both HEA NPs exhibit an excellent light absorption performance of >96%. The grey area presents the solar radiation spectrum (Air mass 1.5). The noisy signal above 2000 nm is due to water absorption bands, mostly in air. The discontinuity of the signal around 800 nm is caused by a detector change in the spectrophotometer.